How do I use the broker???

There are generally two ways to use the Broker in Pokari
1. Sell Ebucks
2. Buy Ebucks

Talk to the Broker, then click Etherbuck Trade
The left Column of the window will show Ebucks that are being sold for a stated rate, and trying to be bought for a stated rate. Ebucks being sold are listed in Yellow numbers, while the ebucks in the more 'wanted' section are in Green numbers.

The upper right section of the window allows you to either sell or buy ebucks.
The lower right section shows how many ebucks and gold you already have being held by the broker.

To buy Ebucks, you'll need to Deposit the gold from your bag into the Broker, then use that gold to complete the Upper Right section when the Buy tab is pressed in. You can spend as low as 1b for an Ebuck, or as high as 9999g, but it's better to keep it in the general range you see other people listing for. Usually this range doesn't exceed 100g from lowest to highest charges. You'll also have to keep in mind that the person who tries to sell ebucks to the Buying section will always sell to the person at the top of the list first. There is no way for him to sell to a person buying for lower. So being at the top of the list is more likely to put the Ebucks in your hand.
You can also check the Log button at the bottom left of the window to see all recent sales of Ebucks, how much and how many have been sold, and so on.

To sell Ebucks, you can either directly sell to the people in the buying section (Green) by selling the ebucks for no higher than the top bid, or you can sell for higher and have it sit in the Selling section (Yellow). Selling to the buying section makes the transaction instant, where you'll have to wait for someone willing to pay higher if you ask for more.

I fear I didn't realize how complex this is to explain .-.
Other facts:
You cannot have over 20kg in the Broker at one time
You can Buy/Sell ebucks for as many different rates at the same time as you can afford
You can put a password on the Broker if you want to (This requires you Unlock it every time you try to make a transaction)
The broker and all your funds in it are shared equally between every character on one account, so you can access it from any one of them.
You'll need to Withdraw the Ebucks from the Broker before you can make purchases from the CS
Ebucks are not tradeable between players, the CS is the only way to transfer them around.

Just a note you can deposit more then 20k gold so if you have a lot of gold it is better to store it all on the broker and then you can access it from your main and all your alts on that account if you need.